What are the tiers of ores in Terraria?

A world will generally only contain one Ore type from each tier, which are chosen at random (see tables below). The alternate Ore types that don't generate naturally in a world can be obtained from Slimes, Crates, an Extractinator, or from other worlds. Hardmode Ore types are chosen at random when players destroy Altars. read more

Lower-tier Ores can be found among Dirt near the Surface, or within Floating Islands. Some Ores are also dropped by bosses, namely Eye of Cthulhu, Eater of Worlds, Brain of Cthulhu, Ocram, and Moon Lord. read more

Standard Ores can be found almost everywhere underground: Copper or Tin; Iron or Lead; Silver or Tungsten; Gold or Platinum; Demonite or Crimtane; Demonite and Crimtane are rarely generated, so that's why The Eater of Worlds and the The Brain of Ctulhu drops them in large quantities. Hardmode Ores are generated by destroying Demon Altars with a Pwnhammer. read more

Instead, all such ores will be spawned in different ways once hardmode is activated in a given world. Small amounts of Cobalt Ore, Mythril Ore, Adamantite Ore, and their alternatives will be placed throughout a world based on how many Demon Altars or Crimson Altars are destroyed by the player. read more